Re: [Qemu-devel] [PATCH v2] Kconfig: add documentation

[Liam Merwick <liam.merwick@oracle.com>]

On 12/02/2019 09:57, Paolo Bonzini wrote:
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Reviewed-by: Liam Merwick <liam.merwick@oracle.com>

> ---
>   docs/devel/kconfig.rst | 305 +++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 305 insertions(+)
>   create mode 100644 docs/devel/kconfig.rst
> 
> diff --git a/docs/devel/kconfig.rst b/docs/devel/kconfig.rst
> new file mode 100644
> index 0000000000..ff1ed3d1b2
> --- /dev/null
> +++ b/docs/devel/kconfig.rst
> @@ -0,0 +1,305 @@
> +Introduction
> +------------
> +
> +QEMU is a very versatile emulator; it can be built for a variety of
> +targets, where each target can emulate various boards and at the same
> +time different targets can share large amounts of code.  For example,
> +a POWER and an x86 board can run the same code to emulate a PCI network
> +card, even though the boards use different PCI host bridges, and they
> +can run the same code to emulate a SCSI disk while using different
> +SCSI adapters.  ARM, s390 and x86 boards can all present a virtio-blk
> +disk to their guests, but with three different virtio guest interfaces.
> +
> +Each QEMU target enables a subset of the boards, devices and buses that
> +are included in QEMU's source code.  As a result, each QEMU executable
> +only links a small subset of the files that form QEMU's source code;
> +anything that is not needed to support a particular target is culled.
> +
> +QEMU uses a simple domain-specific language to describe the dependencies
> +between components.  This is useful for two reasons:
> +
> +* new targets and boards can be added without knowing in detail the
> +  architecture of the hardware emulation subsystems.  Boards only have
> +  to list the components they need, and the compiled executable will
> +  include all the required dependencies and all the devices that the
> +  user can add to that board;
> +
> +* users can easily build reduced versions of QEMU that support only a subset
> +  of boards or devices.  For example, by default most targets will include
> +  all emulated PCI devices that QEMU supports, but the build process is
> +  configurable and it is easy to drop unnecessary (or otherwise unwanted)
> +  code to make a leaner binary.
> +
> +This domain-specific language is based on the Kconfig language that
> +originated in the Linux kernel, though it was heavily simplified and
> +the handling of dependencies is stricter in QEMU.
> +
> +Unlike Linux, there is no user interface to edit the configuration, which
> +is instead specified in per-target files under the ``default-configs/``
> +directory of the QEMU source tree.  This is because, unlike Linux,
> +configuration and dependencies can be treated as a black box when building
> +QEMU; the default configuration that QEMU ships with should be okay in
> +almost all cases.
> +
> +The Kconfig language
> +--------------------
> +
> +Kconfig defines configurable components in files named ``hw/*/Kconfig``.
> +Note that configurable components are _not_ visible in C code as preprocessor
> +symbols; they are only visible in the Makefile.  Each configurable component
> +defines a Makefile variable whose name starts with ``CONFIG_``.
> +
> +All elements have boolean (true/false) type; truth is written as ``y``, while
> +falsehood is written ``n``.  They are defined in a Kconfig
> +stanza like the following::
> +
> +      config ARM_VIRT
> +         bool
> +         imply PCI_DEVICES
> +         imply VFIO_AMD_XGBE
> +         imply VFIO_XGMAC
> +         select A15MPCORE
> +         select ACPI
> +         select ARM_SMMUV3
> +
> +The ``config`` keyword introduces a new configuration element.  In the example
> +above, Makefiles will have access to a variable named ``CONFIG_ARM_VIRT``,
> +with value ``y`` or ``n`` (respectively for boolean true and false).
> +
> +Boolean expressions can be used within the language, whenever ``<expr>``
> +is written in the remainder of this section.  The ``&&``, ``||`` and
> +``!`` operators respectively denote conjunction (AND), disjunction (OR)
> +and negation (NOT).
> +
> +The ``bool`` data type declaration is optional, but it is suggested to
> +include it for clarity and future-proofing.  After ``bool`` the following
> +directives can be included:
> +
> +**dependencies**: ``depends on <expr>``
> +
> +  This defines a dependency for this configurable element. Dependencies
> +  evaluate an expression and force the value of the variable to false
> +  if the expression is false.
> +
> +**reverse dependencies**: ``select <symbol> [if <expr>]``
> +
> +  While ``depends on`` can force a symbol to false, reverse dependencies can
> +  be used to force another symbol to true.  In the following example,
> +  ``CONFIG_BAZ`` will be true whenever ``CONFIG_FOO`` is true::
> +
> +    config FOO
> +      select BAZ
> +
> +  The optional expression will prevent ``select`` from having any effect
> +  unless it is true.
> +
> +  Note that unlike Linux, QEMU will detect contradictions between
> +  ``depends on`` and ``select`` statements and prevent you from building
> +  such a configuration.
> +
> +**default value**: ``default <value> [if <expr>]``
> +
> +  Default values are assigned to the config symbol if no other value was
> +  set by the user via ``default-configs/*.mak`` files, and only if
> +  ``select`` or ``depends on`` directives do not force the value to true
> +  or false respectively.
> +
> +  ``<value>`` can be ``y`` or ``n``; it cannot be an arbitrary Boolean
> +  expression.  However, a condition for applying the default value
> +  can be added with ``if``.  A config option can have any number of
> +  default values (usually, if more than one default is present, they
> +  will have different conditions). If multiple default values satisfy
> +  their condition, only the first defined one is active.
> +
> +**reverse default** (weak reverse dependency): ``imply <symbol> [if <expr>]``
> +
> +  This is similar to ``select`` as it applies a lower limit of ``y``
> +  to another symbol.  However, the lower limit is only a default
> +  and the "implied" symbol's value may still be set to ``n`` from a
> +  ``default-configs/*.mak`` files.  The following two examples are
> +  equivalent::
> +
> +    config FOO
> +      bool
> +      imply BAZ
> +
> +    config BAZ
> +      bool
> +      default y if FOO
> +
> +  The next section explains where to use ``imply`` or ``default y``.
> +
> +Guidelines for writing Kconfig files
> +------------------------------------
> +
> +Configurable elements in QEMU fall under five broad groups.  Each group
> +declares its dependencies in different ways:
> +
> +**subsystems**, of which **buses** are a special case
> +
> +  Example::
> +
> +    config SCSI
> +      bool
> +
> +  Subsystems always default to false (they have no ``default`` directive)
> +  and are never visible in ``default-configs/*.mak`` files.  It's
> +  up to other symbols to ``select`` whatever subsystems they require.
> +
> +  They sometimes have ``select`` directives to bring in other required
> +  subsystems or buses.  For example, ``AUX`` (the DisplayPort auxiliary
> +  channel "bus") selects ``I2C`` because it can act as an I2C master too.
> +
> +**devices**
> +
> +  Example::
> +
> +    config MEGASAS_SCSI_PCI
> +      bool
> +      default y if PCI_DEVICES
> +      depends on PCI
> +      select SCSI
> +
> +  Devices are the most complex of the five.  They can have a variety
> +  of directives that cooperate so that a default configuration includes
> +  all the devices that can be accessed from QEMU.
> +
> +  Devices *depend on* the bus that they lie on, for example a PCI
> +  device would specify ``depends on PCI``.  An MMIO device will likely
> +  have no ``depends on`` directive.  Devices also *select* the buses
> +  that the device provides, for example a SCSI adapter would specify
> +  ``select SCSI``.  Finally, devices are usually ``default y`` if and
> +  only if they have at least one ``depends on``; the default could be
> +  conditional on a device group.
> +
> +  Devices also select any optional subsystem that they use; for example
> +  a video card might specify ``select EDID`` if it needs to build EDID
> +  information and publish it to the guest.
> +
> +**device groups**
> +
> +  Example::
> +
> +    config PCI_DEVICES
> +      bool
> +
> +  Device groups provide a convenient mechanism to enable/disable many
> +  devices in one go.  This is useful when a set of devices is likely to
> +  be enabled/disabled by several targets.  Device groups usually need
> +  no directive and are not used in the Makefile either; they only appear
> +  as conditions for ``default y`` directives.
> +
> +  QEMU currently has two device groups, ``PCI_DEVICES`` and
> +  ``TEST_DEVICES``.  PCI devices usually have a ``default y if
> +  PCI_DEVICES`` directive rather than just ``default y``.  This lets
> +  some boards (notably s390) easily support a subset of PCI devices,
> +  for example only VFIO (passthrough) and virtio-pci devices.
> +  ``TEST_DEVICES`` instead is used for devices that are rarely used on
> +  production virtual machines, but provide useful hooks to test QEMU
> +  or KVM.
> +
> +**boards**
> +
> +  Example::
> +
> +    config SUN4M
> +      bool
> +      imply TCX
> +      imply CG3
> +      select CS4231
> +      select ECCMEMCTL
> +      select EMPTY_SLOT
> +      select ESCC
> +      select ESP
> +      select FDC
> +      select SLAVIO
> +      select LANCE
> +      select M48T59
> +      select STP2000
> +
> +  Boards specify their constituent devices using ``imply`` and ``select``
> +  directives.  A device should be listed under ``select`` if the board
> +  cannot be started at all without it.  It should be listed under
> +  ``imply`` if (depending on the QEMU command line) the board may or
> +  may not be started without it.  Boards also default to false; they are
> +  enabled by the ``default-configs/*.mak`` for the target they apply to.
> +
> +**internal elements**
> +
> +  Example::
> +
> +    config ECCMEMCTL
> +      bool
> +      select ECC
> +
> +  Internal elements group code that is useful in several boards or
> +  devices.  They are usually enabled with ``select`` and in turn select
> +  other elements; they are never visible in ``default-configs/*.mak``
> +  files, and often not even in the Makefile.
> +
> +Writing and modifying default configurations
> +--------------------------------------------
> +
> +In addition to the Kconfig files under hw/, each target also includes
> +a file called ``default-configs/TARGETNAME-softmmu.mak``.  These files
> +initialize some Kconfig variables to non-default values and provide the
> +starting point to turn on devices and subsystems.
> +
> +A file in ``default-configs/`` looks like the following example::
> +
> +    # Default configuration for alpha-softmmu
> +
> +    # Uncomment the following lines to disable these optional devices:
> +    #
> +    #CONFIG_PCI_DEVICES=n
> +    #CONFIG_TEST_DEVICES=n
> +
> +    # Boards:
> +    #
> +    CONFIG_DP264=y
> +
> +The first part, consisting of commented-out ``=n`` assignments, tells
> +the user which devices or device groups are implied by the boards.
> +The second part, consisting of ``=y`` assignments, tells the user which
> +boards are supported by the target.  The user will typically modify
> +default the configuration by uncommenting lines in the first group,
> +or commenting out lines in the second group.
> +
> +It is also possible to run QEMU's configure script with the
> +``--with-default-devices`` option.  When this is done, everything defaults
> +to ``n`` unless it is ``select``ed or explicitly switched on in the
> +``.mak`` files.  In other words, ``default`` and ``imply`` directives
> +are disabled.  When QEMU is built with this option, the user will probably
> +want to change some lines in the first group, for example like this::
> +
> +   CONFIG_PCI_DEVICES=y
> +   #CONFIG_TEST_DEVICES=n
> +
> +and/or pick a subset of the devices in those device groups.  Right now
> +there is no single place that lists all the optional devices for
> +``CONFIG_PCI_DEVICES`` and ``CONFIG_TEST_DEVICES``.  In the future,
> +we expect that ``.mak`` files will be automatically generated, so that
> +they will include all these symbols and some help text on what they do.
> +
> +``Kconfig.host``
> +----------------
> +
> +In some special cases, a configurable element depends on host features
> +that are detected by QEMU's configure script; for example some devices
> +depend on the availability of KVM or on the presence of a library on
> +the host.
> +
> +These symbols should be listed in ``Kconfig.host`` like this::
> +
> +    config KVM
> +      bool
> +
> +and also listed as follows in the top-level Makefile's ``MINIKCONF_ARGS``
> +variable::
> +
> +    MINIKCONF_ARGS = \
> +      $@ $*-config.devices.mak.d $< $(MINIKCONF_INPUTS) \
> +      CONFIG_KVM=$(CONFIG_KVM) \
> +      CONFIG_SPICE=$(CONFIG_SPICE) \
> +      CONFIG_TPM=$(CONFIG_TPM) \
> +      ...
>